Method and Device for Processing a Video Sequence

Abstract

The invention concerns a method and a device ( 10, 20 ) for processing a video sequence ( 101 ) constituted by images composed of blocks of coefficients, and comprising the steps of: generating ( 511 ) first and second reconstructions ( 402 to 413 ) of a (same) first image, to obtain first and second reference images ( 517 ), the second reconstruction implementing, on a coefficient of a block, a different operation to that of the first reconstruction, predicting ( 505 ) a part ( 414, 415, 416 ) of said current image ( 401 ) on the basis of one of the reference images, wherein the second reconstruction comprises: the obtainment, for a block of the first image, values (W i ) calculated on the basis of the block coefficients and representing spatial frequency information; the selection of a coefficient according to these calculated values, to apply to it said different operation and obtain said second reference image.

1 . A method of processing a video sequence ( 101 ,  201 ,  501 ,  601 ,  701 ,  801 ) constituted by a series of digital images ( 401  to  407 ) comprising a current image ( 401 ) to process, said images being composed of data blocks each formed from a set of coefficients each taking a value, said method comprising the steps of:
 generating ( 511 ,  603 ,  720 ,  812 ,  519 ,  612 ,  720 ,  812 ) at least first and second reconstructions ( 402  to  413 ), which are different from each other, of at least a same first image (i- 1  to i-n) of the sequence, so as to obtain at least first and second reference images ( 517 ,  610 ,  717 ,  810 ,  518 ,  611 ,  718 ,  811 ), the second reconstruction implementing, on at least one coefficient of a block, a different operation to that implemented at the first reconstruction on the same block coefficient; 
 predicting ( 505 ,  606 ,  705 ,  806 ) at least a part ( 414 ,  415 ,  416 ) of said current image ( 401 ) on the basis of at least one of said reference images ( 516 ,  608 ,  716 ,  808 ) 
 and wherein the second reconstruction of the first image comprises the steps of: 
 obtaining, for at least one block of the first image, values (W i , G X   i,j , G Y   i,j , G i,j ) calculated on the basis of the block coefficients and representing spatial frequency information; 
 selecting at least one block coefficient according to said calculated values, and applying said different operation to it so as to obtain said second reference image. 
 
     
     
         2 . A method according to  claim 1 , wherein the second reconstruction of the first image further comprises a step of determining a subset (I′) of block coefficients, according to said calculated values;
 and selecting at least one block coefficient is carried out on said subset. 
 
     
     
         3 . A method according to  claim 2 , wherein determining said subset comprises calculating, for each block coefficient and on the basis of said calculated values, at least a second value (Sum i ) representing the relative importance of the block coefficients compared with each other within said first image, and determining coefficients to constitute said subset by comparison of the second values with a threshold value (max/Δ). 
     
     
         4 . A method according to  claim 2 , comprising a plurality of different reconstructions of the first image, and wherein the second reconstruction of the first image further comprises the step of deleting, from said subset, a coefficient on which said different operation has already been applied for a predefined number of reconstructions of the first image. 
     
     
         5 . A method according to  claim 1 , wherein, given a selected coefficient, the second reconstruction comprises applying the different operation to said selected coefficient for each block of the first image to reconstruct. 
     
     
         6 . A method according to  claim 1 , wherein, at the second reconstruction, the steps to obtain said calculated values and to select at least one coefficient are carried out repetitively for several blocks of the first image, so as to obtain a particular coefficient for each of those several blocks, and, in a said block, an operation is applied to the particular coefficient which operation is different to that implemented at the first reconstruction on the same block coefficient. 
     
     
         7 . A method according to  claim 6 , wherein, at the second reconstruction, a said different operation is applied to at least one said first coefficient for each block of the first image to reconstruct and a said different operation is applied, for each block of the first image, to a particular selected coefficient of said block,
 and selecting said particular coefficient before applying said different operation is carried out from a set of block coefficients excluding said at least one first coefficient.   
     
     
         8 . A method according to  claim 1 , wherein the calculated values representing spatial frequency information are obtained by transforming the first reference image resulting from the first reconstruction. 
     
     
         9 . A method according to  claim 1 , wherein the first reconstruction comprises inverse quantization of the coefficient values coming from a quantized version of said first image, and
 for the second reconstruction, said calculated values are calculated on the basis of the dequantized values of coefficients obtained at the inverse quantization of the first reconstruction.   
     
     
         10 . A method according to  claim 1 , wherein said calculated values are calculated for each block coefficient and represent a gradient in the neighborhood of the block coefficient. 
     
     
         11 . A method according to  claim 1 , wherein the operation that is different from the second reconstruction implements an inverse quantization using, for said at least one selected block coefficient (W i ), a different quantization offset (θ i ) to that used for the same coefficient at the first reconstruction. 
     
     
         12 . A method according to  claim 1 , wherein, for the set of the blocks composing a first given image, there is uniquely associated with a block coefficient a parameter (θ I ) defining said different operation to apply, for each block, to the block coefficient if it is selected,
 and the method comprises a step of coding the current image into a coded stream ( 510 ,  710 ), and a step of inserting, into the coded stream, said parameters for the set of the blocks of a current image, in the form of a unique list organized in a predetermined order. 
 
     
     
         13 . A device for processing ( 10 ,  20 ) a video sequence ( 101 ,  201 ,  501 ,  601 ,  701 ,  801 ) constituted by a series of digital images ( 401  to  407 ) comprising a current image ( 401 ) to process, said images being composed of data blocks each formed from a set of coefficients each taking a value, said device comprising:
 a generating means adapted for generating at least first and second reconstructions ( 402  to  413 ), which are different from each other, of at least a same first image (i- 1  to i-n) of the sequence, so as to obtain at least first and second reference images ( 517 ,  610 ,  717 ,  810 ,  518 ,  611 ,  718 ,  811 ), the second reconstruction implementing, on at least one coefficient of a block, a different operation to that implemented at the first reconstruction on the same block coefficient; 
 a predicting means adapted for predicting at least a part ( 414 ,  415 ,  416 ) of said current image ( 401 ) on the basis of at least one of said reference images ( 516 ,  608 ,  716 ,  808 ) 
 and wherein the generating means is configured, for generating the second reconstruction, for: 
 a means for obtaining, for at least one block of the first image, values (W i , G X   i,j , G Y   i,j , G i,j ) calculated on the basis of the block coefficients and representing spatial frequency information; 
 a means for selecting at least one block coefficient according to said calculated values, and applying said different operation to it so as to obtain said second reference image. 
 
     
     
         14 . A means of information storage, possibly totally or partially removable, that is readable by a computer system, comprising instructions for a computer program adapted to implement the processing method according to  claim 1 , when the program is loaded and executed by the computer system. 
     
     
         15 . A computer program product readable by a microprocessor, comprising portions of software code adapted to implement the processing method according to any  claim 1 , when it is loaded and executed by the microprocessor.